A differential transmission system is known as one of the transmission systems for transmitting a digital signal between electronic devices. The differential transmission system is a system in which digital signals each having opposite phases are inputted in a single pair of signal lines, and external noise generated from the signal lines or radiation noise is cancelled out through differential transmission. Since noise is reduced by the cancellation of external noise, a signal can be transmitted at a low amplitude. This system has further advantages in that the amplitude of the signal is reduced, the rising and falling time of the signal are reduced, and the speed of signal transmission can therefore be increased.
Interface specifications that use this differential transmission system include USB (Universal Serial Bus), IEEE1394, LVDS (Low Voltage Differential Signaling), DVI (Digital Video Interface), HDMI (High-Definition Multimedia Interface), and other specifications. Among these specifications, HDMI is a high-speed digital interface that enables more digital signals to be transmitted, and that enables uncompressed digital signals to be transmitted between a source device (e.g., a DVD player, a set-top box, or the like) and a sink device (e.g., a digital television, a projector, or the like). According to HDMI, video signals and audio signals can be transmitted at high speed by a single HDMI cable.
A common mode choke coil is used as a noise suppressor in a high-speed differential transmission line. Although a differential transmission system has the characteristics of strong external noise suppression and no generation of noise, common mode noise is actually generated by slight imbalances of the two signals, and such noise is emitted from interface cables and the like. A common mode choke coil is most effective at removing this noise. Small-sized, high-performance thin-film-type common mode choke coils have been preferred for use, particularly in recent times (see Japanese Laid-open Patent Application No. H08-203737).
In HDMI and other high-speed interfaces, the structure of the ICs as such has become vulnerable to ESD (Electrostatic Discharge) as speed has been increased. Therefore, the need for ESD protection in high-speed transmission ICs is increasing, and varistors, Zener diodes, and other capacitive elements are used as ESD protection components.
However, when a capacitive element is inserted as an ESD protection component in a transmission channel, drawbacks occur in that the signals transmitted through the transmission channel, particularly high-frequency signals (200 MHz or higher) or high-speed pulse signals, are reflected and attenuated. The reason for this is that when a capacitive element is inserted in the transmission channel, the characteristic impedance in the position at which the capacitive element is inserted is reduced by the capacitance component of the capacitive element, and the impedance is mismatched at the insertion position. When there is a portion of the transmission channel in which the impedance is mismatched, the high-frequency component of the signal causes reflection in the impedance-mismatched portion, and return loss therefore occurs. As a result, the signal is significantly attenuated. The reflection also sometimes causes unwanted radiation in the transmission channel, which causes noise. In HDMI, the specified value (TDR specification) of the characteristic impedance of the transmission line is 100Ω±15% (High-Definition Multimedia Interface Specification Version 1.1).
As a result of concentrated investigation of signal transmission circuits capable of suppressing a reduction in the characteristic impedance even when a capacitive element is used for ESD protection, the inventors discovered that a reduction of characteristic impedance is effectively suppressed by providing a common mode choke coil, and an inductor that is substantially not magnetically coupled with an inductor included in the common mode choke coil, in a state in which the common mode choke coil and the inductor are connected in series to each other in a stage prior to a capacitive element. Furthermore, including a characteristic impedance adjustment coil in the common mode choke coil makes it possible to reduce the number of components, lower the cost, and enhance reliability (see Japanese Laid-open Patent Application No. 2006-140229).
However, merely adding a characteristic impedance adjustment coil to the common mode choke coil is inadequate, and there is a need for a characteristic impedance adjustment coil having satisfactory characteristics in which there is minimal fluctuation in the inductor values of two characteristic impedance adjustment coils, and in which magnetic coupling between inductors is adequately suppressed.